A conventional data transmission system as illustrated by a circuit diagram in FIG. 4(A) comprises multiple electronic equipment 10, 20 each provided with an individual power supply, and a cable 30 for connecting therebetween. The data transmission system includes two sets of data transmitting circuits each having an open drain type output section at a transmitting end and an input section provided with a pull-up type resistor at a receiving end, and these data transmitting circuits extend between the electronic equipment 10, 20 and the cable 30. A first digital data transmitting circuit is provided for transmitting data from the electronic equipment 10 to the electronic equipment 20 while a second digital data transmitting circuit is provided for transmitting data from the electronic equipment 20 to the electronic equipment 10 in an opposite direction.
The electronic equipment 10 (first electronic equipment) comprises a power supply 11 of an output voltage Va, an internal circuit 14 for effecting various data processing adaptable to intend applications, a transmitting circuit 12 for subjecting transmission data supplied from the internal circuit 14 to a proper coding processing or modulating processing such as a parallel-serial conversion and the like so as to adjust a data format, an open drain type output section 13 (first output section) for binarizing an output signal from the transmitting circuit 12 and transmitting the binarized output signal to the electronic equipment 20 by way of the cable 30, an input section 16 (second input section) for waveform shaping a binary signal supplied from the electronic equipment 20 by way of the cable 30 and inputting thereto, a pull-up resistor Ra (second resistor) which is attached to the input side of the input section 16, and a receiving circuit 15 for subjecting the received data inputted to the input section 16 to a proper decoding processing or demodulating processing such as serial-parallel conversion and the like so as to adjust a data format and supplying it to the internal circuit 14.
The transmitting circuit 12, the internal circuit 14, the receiving circuit 15, the input section 16 are respectively operated under the output voltage Va while the pull-up resistor Ra is connected to a power supply line of the output voltage Va at one end and also connected to an input line of the input section 16 at the other end so as to perform its function utilizing the output voltage Va. On the other hand, the open drain type output section 13 is employed so as to supply a signal without dependence on the output voltage Va of its own electronic equipment 10. That is, the output section 13 is rendered in a ground state in its output when an output value thereof is low while it is rendered in a high impedance state (floating state, high resistive state) in its output when the output value is high.
The electronic equipment 20 (second electronic equipment) that is a communication counterpart of the electronic equipment 10 comprises a power supply 21 of an output voltage Vb, an internal circuit 24 for effecting various data processing adaptable to intend applications, a transmitting circuit 25 for subjecting transmission data supplied from the internal circuit 24 to a proper coding processing or modulating processing such as a parallel-serial conversion and the like so as to adjust a data format, an open drain type output section 26 (second output section) for binarizing an output signal from the transmitting circuit 25 and transmitting the binarized output signal to the electronic equipment 10 by way of the cable 30, an input section 23 (first input section) for waveform shaping a binary signal supplied from the electronic equipment 10 by way of the cable 30 and inputting thereto, a pull-up resistor Rb (first resistor) which is attached to the input side of the input section 23, and a receiving circuit 22 for subjecting the received data inputted to the input section 23 to a proper decoding processing or demodulating processing such as serial-parallel conversion and the like so as to adjust a data format and supplying it to the internal circuit 24.
The input section 23, the receiving circuit 22, the internal circuit 14 and the transmitting circuit 25 are respectively operated under the output voltage Vb while the pull-up resistor Rb is connected to a power supply line of the output voltage Vb at one end and also connected to an input line of the input section 23 at the other end so as to perform its function utilizing the output voltage Vb. On the other hand, an open drain type output section 26 is employed so as to supply a signal without dependence on the output voltage Vb of its own electronic equipment 20. That is, the output section 26 is rendered in ground state in its output when an output value thereof is low while it is rendered in a high impedance state (floating state, high resistive state) in its output when the output value is high.
The cable 30 has a connector 31 provided at its one end at the electronic equipment 10 side and another connector 32 provided at its other end at the electronic equipment 20 side, and also it has a long intermediate portion which is flexible and soft so that a physical connection between the electronic equipment 10 and electronic equipment 20 can be dynamically and simply established in compliance with the necessity of communication. Multiple signal transmission lines 33, 35 and a ground line 34 which are respectively made of copper wire and the like, and insulatively coated, and built in the cable 30. The lines 33, 34, 35 are respectively connected to corresponding contact terminals of the connector 31 at each one end and also connected to corresponding contact terminals of the connector 32 at each other end. The ground line 34 may be connected to a shield or may act as a shield.
When the electronic equipment 10 and the electronic equipment 20 are connected to each other, by the cable 30, the output line of the output section 13, the signal transmission line 33 and the input line of the input section 23 are connected to one another, and the ground line of the electronic equipment 10, the line 34 and the ground line of the electronic equipment 20 are connected to one another while the output line of the output section 26, the line 35 and the input line of the input section 16 are connected to one another. That is, a first digital data transmitting circuit for transmitting a binary signal from the electronic equipment 10 to the electronic equipment 20 is formed of the output section 13, the line 33 and the input section 23, while a second digital data transmitting circuit for transmitting a binary signal from the electronic equipment 20 to the electronic equipment 10 in an opposite direction is formed of the output section 26, the line 35 and the input section 16.
The connector 31 is mounted onto the electronic equipment 10 and the connector 32 is mounted onto the electronic equipment 20, and the electronic equipment 10 and the electronic equipment 20 are connected to each other by the cable 30 so as to transmit data in order to transmit and receive data between the electronic equipment 10, 20. In this state, the transmission of data from the electronic equipment 10 to the electronic equipment 20 is effected by the first digital data transmitting circuit (output section 13→line 33→input section 23), while the transmission of data from the electronic equipment 20 to the electronic equipment 10 is effected by the second digital data transmitting circuit (output section 26→line 35→input section 16).
More in detail, an output state of the output section 13 (or 26) is changed between a ground state and a high impedance state as data value to be transmitted is low or high or ever changed so that the line 33 (or 35) is rendered in the ground state when a low data value is outputted. Such a ground state is inputted to the input section 23 (or 16) so that the low data value is transmitted. On the other hand, when a high data value is outputted, the signal transmission line 33 (or 35) is separated from the ground and is rendered in an output voltage Vb (or Va) application state by way of the pull-up resistor Rb (or Ra) and such a state is inputted to the input section 23 (or 16) so that the high data value is transmitted.
According to the conventional data transmission system, since the open drain type output sections 13, 26 are employed at the transmitting end while the input sections 23, 16 provided with the pull-up resistor Rb, Ra are employed at the receiving end, even if the electronic equipment 10, 20 are provided with individual power supplies 11, 21 and operate by their own power supply voltages Va, Vb, they can be connected to each other so as to transmit data by the cable 30 without being bound by the output voltage Va, Vb of the communication counterpart.
Although line capacitance is intensively illustrated on the lines 33, 35 of the cable 30, a capacitance C which distributes between the ground line 34 and a shield or other coating line or the like is parasitic on the lines 33, 35. The capacitance C has a property to increase substantially in proportion to the lengths of the lines 33, 35, and it is generally considerably larger than the capacitance which is parasitic on each circuit inside equipment. When the signals on the lines 33, 35 are changed from a low state to a high state, they slow down by time constant (resistance Rb×capacitance C), (resistance Ra×capacitance C), in accordance with the combination of the capacitance C, resistors Rb, Ra at rising of the signals.
Accordingly, if the data transmission rate is suitable, it is possible to obtain a signal waveform which clearly shows a binary state (see waveform example at the time of low transmission rate shown in FIG. 4B). If the data transmission rate is increased from the foregoing rate, the signal waveform is collapsed, particularly a high state is not made clear (see the waveform example at the time of high transmission rate shown in FIG. 4C) so that the data is not transmitted accurately, causing a problem that it is difficult to speed up data transmission or increase data transmission rate.
Although various standards and the like capable transmitting data at high speed are proposed and in practical use, the restriction imposed on a cable and a transmitting circuit become severe as the data transmission is speeded up, a driving voltage and other bindings are imposed on a driving condition of the signal transmission line.
Under the circumstances, at present if data is transmitted without being bound by a counterpart's power supply voltage, a transmission rate is restricted, while if data is transmitted by increasing the transmission rate, the data transmission is bound by the counterpart's power supply voltage.
It becomes a technical problem to increase the transmission rate while following an open drain type transmission system adaptable for a different power supply so as to meet both advantages, namely, not to be restricted in transmission rate and not to be bound by the counterpart's power supply voltage.